Eddy current test method and appratus for selecting calibration standard to measure thickness of micro thin film metal coating on wafer products by using noncontact technique

ABSTRACT

A method of measuring a thin film metal coating on wafer products by using eddy current technologies. An absolute shielded probe is used to generate and detect signals in order to measure the resistant path of eddy current that induce into the thin film metal coating on semiconductor wafer products. An alternate magnetic field is adjacent to a conductive material that will generate a closed loop eddy current path. This eddy current is called secondary current which has the same characteristics of a primary current and a magnetic field; such as having the same frequency, having an alternate current mode, always seeking the least resistant path to travel, and producing a secondary alternate magnetic field. An eddy current sensor probe generates a unique characteristic locus of different thickness of the same materials that have the same resistivity. Based on this physical phenomenon, we are able to measure and predict the thickness of micro thin film of the same metal coating on semiconductor wafers very accurately without contact on the semiconductor wafer. A series of different thicknesses of micro thin film metal coating on a wafer will need to produce a series of thickness reference loci. The unknown thickness sample of the same resistivity can then be determined by comparing the two loci adjacent to the upper and lower unknown thickness sample by simple interpolation.

FIELD OF THE INVENTION

[0001] The invention introduces a method of an apparatus for selecting the calibration of different materials to measure the thickness and sheet resistance of different metal coatings on semiconductor wafer products. And the use of digital thickness calibration standards, by way of software algorithm, avoids the constant re-calibration to ensure that there is no thermal drift by electronic components. With this unique software algorithm, we are able to measure the coating thickness from multiple probes by either reading simultaneous probe measurements or multiplexed readings.

BACKGROUND OF THE INVENTION

[0002] As semiconductor wafers increase in size, demands for higher yield output and quality level products, wafer cost and operation, and speed all call the need for nondestructive testing. Whether it is an inline semiconductor process or a standalone unit, nondestructive testing can be used in ways such as the Chemical Machining Process, or measuring and monitoring the thickness of metal deposition on semiconductors. It is well known that the eddy current can be used to measure thickness and conductance of materials. An unknown thickness, which is within the range of a calibration curve, will be found by set of predetermined values. From the calibration curve the unknown sample can be measured by finding the intersected point between the calibration curve and the unknown sample curve. The bottom line is that a good calibration, the most important information, must be generated before actually measuring the unknown sample.

SUMMARY OF THE INVENTION

[0003] The inventive apparatus is capable of measuring the thickness of a metal coating on semiconductor wafer products. Then from a known constant resistivity, sheet resistance can be calculated. It is very easy to obtain accurate thickness using a destructive method. A microscope or an electron microscope can be used to measure the thickness of a coating after the coating is cut through the cross section. The other half will be kept for thickness reference. This is true when the thickness of the thin film metal is above 1 micron or 10,000 Angstroms. If the thickness of metal thin film is below 10,000 Angstroms, then it is a problem of measuring even under electron microscope. Another important fact is that there is no equal operational amplifier, so it is very difficult to avoid the thermal drift of electronic instruments. Therefore, the signal collected from the eddy current is constantly changing during data collection and inspection, in respects to the electronic reference point. This phenomenon is a headache for all engineers who want a consistency in their reading day after day. To ensure the reading is repeatable, a constant re-calibration must be done before actual measurement as short of a time as possible to avoid the shifting of electronic reference point due to thermal drifting.

[0004] To solve this difficulty, obtain several different thicknesses of different materials which have a lower conductivity than the materials required to be measured. This will cover the range of the unknown thickness and software algorithm to offset thermal drift due to hardware problems. For example, select annealed Ti 6-4 material to become the calibration set of thickness for annealed copper. Because the conductivity of annealed Ti-6-4 is measured by I.A.C.S (International Annealed Copper Standard), the annealed copper is measured to be 100 I.A.C.S. The ratio between the conductivity of Ti 6-4 and copper is 100. By knowing this relationship, the thickness of annealed copper is {fraction (1/100)} of Ti 6-4. In other words, 1000 Angstroms thick of annealed copper is equal to (100×1000 Angstroms=100,000 Angstroms) 100,000 Angstroms Ti 6-4. The thicker the material the more easily available it is in the market and the easier it is to measure accurately. Here is theory and mathematics:

ρ=172.41/σ

[0005] Where ρ is resistivity and σ is conductivity in I.A.C.S. unit

ρ=Thickness×Sheet resistance=t×R(s)

[0006] Where thickness is t and R(s) is sheet resistance

R(s)=ρ/t

[0007] From this relationship we can write:

R(s)copper=ρ copper/t copper

R(s)Ti-6-4=ρ Ti6-4/t Ti 6-4

[0008] if Ti 6-4 has a thickness of 100,000 Angstrom and copper has a thickness of 1,000 Angstroms, then by substituting these values into the above equation we have:

R(s)copper=(171.41/σ)/1000 Angstrom

R(s)copper=(171.41/100 I.A.C.S.)/1000 Angstroms=171.41/100,000 Angstroms (I.A.C.S)

R(s)Ti 6-4 (171.41/1 I.A.C.S.)/100,000 Angstroms=171.41/100,000 Angstroms (I.A.C.S)

[0009] Therefore, R(s)copper=R(s) Ti 6-4 when the thickness of Ti6-4 is 100,000 Angstrom and the thickness of copper is 1,000 Angstroms.

[0010] By knowing this electrical behavior in terms of equivalent sheet resistance, we can select a thicker thickness of Ti 6-4 having a ratio 100 times thicker than copper. To represent the actual thickness of copper we have to use a calibration, which is less than even 100 Angstroms. The following annealed Ti 6-4 thickness is selected to represent the actual thickness of annealed copper: 10000, 20000, 30000, 40000, 50000, 60000, 70000, 80000, 90000, and 100000 Angstroms. These Ti 6-4 thicknesses represent an eddy current response, respectively, of 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 Angstroms of copper. These represent the thicknesses of copper deposition on silicon wafers without the use of cutting to measure the actual thicknesses under an electron microscope. And as it is stated above, if the thickness of metal thin film is under 100 Angstroms, it is difficult to measure.

[0011] BRIEF DESCRIPTION OF THE DRAWING

[0012]FIG. 1 is a simplified schematic diagram of the preferred embodiment of this invention.

[0013]FIG. 2 is a graph of the loci of different materials' known thicknesses. They represent 500, 1000, 1500, 1700, 2000 Angstroms of copper and 50000, 100000, 150000, 170000, 200000 Angstroms of Ti 6-4. They are identical.

[0014]FIG. 3 is a natural triggered gate curve.

[0015]FIG. 4 is a digital calibration curve thickness versus Y value intercepting point from known thickness

[0016]FIG. 5 is a detailed Schematic diagram

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0017] The value of metal thickness increases as sheet resistance decreases, following the simple relationship: ρ=Thickness×Sheet resistance, where ρ is resistivity. The invention prefers to use thickness value for calibration rather than sheet resistance because the invention intends to use different measurement methods to obtain the precise set of calibration. If sheet resistance is used for calibration, the sheet resistance will be measured by another alternative direct electrical method, which can contribute more error. For instance, sheet resistance value can be obtained by a four-points probe when you have a fixed distance from each point contact. Constant voltage and current are applied at two outside pins. Voltage drop is measured at two middle pins. With voltage drop measured and current value known, the sheet resistance can be obtained by the simple equation: V=I×R. This is a simple way to obtain the sheet resistance, but if we look very closely there are many variables that make the measurement of sheet resistance have error. Such as point contact resistance; voltage cannot be absolutely constant; the surface of sample is not flat etc . . . The expression AC Voltage driver, used throughout the specification, is the energy used to drive the inductor coil or eddy current probe at specific frequencies. The sinusoidal wave is preferred to obtain the maximum phase separation between two closest thicknesses.

[0018] A preferred embodiment of the inventive apparatus will be described with reference to FIG. 1. As shown in FIG. 1, the apparatus includes two single absolute eddy current probes comprising housing (3); one sense coil (1) mounted at the end of housing (3); and a reference coil (1) mounted within housing (3), but far away from sense coil (1). Both sense coil (1) and reference coil (2) have the same size of ferrite core, number of coil turns, and a magnetic cable size # 40 gauge. Both coils should be fabricated so that the inductance values are equal at a 10 MHz frequency response. And the resistance of each coil must be less than 20 ohms.

[0019] In FIG. 1, the vertical axis is normal to the surface sample. A vertical axis, such as linear motion control, provides a translating probe housing (3) up and down relative to the position of the sample. The probe housing is mounted with the capacitance probe (20) to keep a constant distance ‘d’ to calibrate the thickness of the sample. Because the capacitance probe (20) is acting as a proximity sensor, the sense coil is always a ‘d’ distance to the surface of the calibration sample, eliminating the need to have an optical microscope maintain a fixed ‘d’ to collect the eddy current data at the same height. Instead, a linear motion control is provided for to withdrawal or come down to very close to the surface sample to ensure the maximum magnitude of eddy current signal without touching the surface. An AC voltage source applied to the sense and reference coils is called a voltage driver.

[0020] The selection of frequency, gain, and voltage driver is needed to obtain the maximum magnitude signal response of eddy current, but still maintain the ability to phase separation at different thicknesses. For a given material, such as copper, we can choose Ti 6-4 material to use for the calibration of copper because it is easy to obtain the thicker thickness. Having a specific frequency, gain, voltage driver level, these shall be selected to have a maximum phase separation and amplitude of eddy current signals for two close thicknesses, for example, 500 and 1000 Angstrom. An AC voltage (6) with pre-selected sinusoidal waves of frequency 10 MHz or higher induces to a Wheatstone Bridge (7) which contains a sense eddy current coil (1) and a reference coil (2). An adjustable electronic bridge circuit (8) is applied to the Wheatstone Bridge to balance the Wheatstone Bridge circuit (7) to zero reference voltage. As the sense eddy current coil (1) comes close to the surface metal coating on the wafer by a translating linear motion control axis, the Wheatstone Bridge (7) unbalances voltage between legs. The voltage is measured. This voltage measures the amplitude of both the in phase component and the quadrature component. This is the result of the X and Y components in the impedance plane display (19). Preferably an A to D board (22) is used to convert the voltage analog signal to Digital signal (22) with software to generate data and store data to a hard-drive and SRAM (Static Random Access Memory) (10) for further processing, calculating (9) and displaying (19) if necessary

[0021] The process of making digital calibration standard curves in FIG. 4 deals with avoiding thermal drift problems. A preferred embodiment of the inventive method will next be described with reference to FIG. 2. The first step of the method is to generate the natural intercepting curve (22) in FIG. 3 (by operating processor and storing data in memory). The second step is to generate the natural intercepting curve equation by using second order polynomial best-fit method. This natural intercepting curve will act as a trigger gate to collect data point (X, Y) at a specific distance above the target. The specific distance is specified at ‘d’.

[0022] A brief discussion about how to maintain a constant ‘d’ above the inspection target. Capacitance sensors produce maximum voltage in air when there is no interference or interruption in charge path from negative to positive poles. As the capacitance sensor (20) comes close to the target inspection, the charge experiences interference, which produces a voltage drop. By selecting specific voltage output from the capacitance sensor (20), a specific distance, or a constant ‘d’, shall be maintained.

[0023] The third step is to store this equation in memory for use on subsequent measurements of the intercepting point on samples having both known and unknown thicknesses.

[0024] The first step in generating the natural intercepting curve is to perform eddy current measurements at points of non-contact on each of a number of samples (N samples) having known thicknesses. Procedure: bring the sense eddy current coil (1) very close to known thickness sample surface. The capacitance probe (20) will ensure non-contact action between eddy current coil and surface of known thickness sample about 75 microns and a capacitance (20) output of 5 volts. The processor (9) will record the X and Y voltage value of N's surface for known thickness. They are shown in FIG. 2.

[0025] Each point is measured for both X and Y components when sense eddy current coil (1) is at a non-contact position to the surface of sample at ‘d’ according to a 5 volt output from the capacitance sensor (20). The value of X and Y voltages are stored in memory (10) for later use.

[0026] The coordinate (X, Y) of these 4 points of known thickness sample A, B, C, and D will generate a natural intercepting curve equation by using exponential best fit procedure from special software. This equation will be in the general form: Y=m e^(−nX). This equation will be used as a triggered gate to record data of the precise distance ‘d’ of all known thicknesses intersecting a point. The m and n coefficients will be calculated by plugging in the value of the (X, Y) voltage pair into the equation Y=m e^(−nX). From sample ‘A’ thickness, two data points are needed to generate and find a linear equation. The first point of known thickness is above the sample ‘A’ at distance ‘d’. The data is collected when the capacitance (20) produces a 5-volt output. The second data point is collected when the Z linear motion axis pulls either the eddy current probe faraway or the sample ‘A’ faraway from the eddy current probe to the position where there is no signal generated from the eddy current to the sample ‘A’. This is called signal of eddy current on air. From these two data points at different positions, two coordinates (X, Y) can generate a linear equation in the form of Y=aX+b. The coefficient ‘b’ is the offset value at that particular short time of collecting data (normally the collection of these two data points is less than 1 second) or the offset value is different from eddy current probe to probe. Coefficient ‘a’ and ‘b’ will be calculated by plugging the value of the (X, Y) voltage pair into the equation. Since the offset value ‘b’ comes from either thermal drift or eddy current probe differences, it will not be a major factor to contribute to error in calibration and measurement from time to time and from probe to probe. Since software instructs to eliminate coefficient ‘b’, the equation will be Y=aX, where ‘a’ is a slope. This equation of sample ‘A’ will be brought back to the original coordinates of the impedance plane at (0,0). From the original coordinates of the impedance plane, find the intersecting point of known thickness sample ‘A’ by setting the natural triggered gate curve to the simplified equation of ‘A’: m e^(−nX)=aX. All ‘m’, ‘n’, and ‘a’ coefficients are calculated above. The value of X will be used to calculate Y for this invention to determine the correct ‘A’ point (X, Y) voltage pair intersecting point. Repeat this step for sample ‘B’, ‘C’, ‘D’ and so on if we have more known thickness samples. After all the value Y of intersecting point of ‘A’, ‘B’, ‘C’, and ‘D’ are found.

[0027] A thickness versus Y coordinate curve is generated; this is a digital calibration thickness curve. This digital calibration thickness curve is used to predict the unknown thickness sample. The equation will be stored under the memory of the computer to predict unknown thicknesses.

[0028] To predict or measure the unknown thickness sample, repeat the process as above to find the intersecting point Y by generating two data points. First, collect data point at ‘d’ distance. Second, collect data point at eddy current on air, which is far away from the unknown thickness target. Third generate equation in the form of Y=aX+b. Then eliminate coefficient ‘b’. The equation will be Y=aX. From the original coordination of the impedance plane, find the intersecting point by solving the equations between Y=aX and the natural triggered gate curve, Y=m e^(−nX). Find the Y intersecting value, and then plug this Y value into the digital calibration curve. A simple interpolation is calculated to find the thickness of the unknown when the unknown thickness is in between two known thicknesses.

[0029] Various modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. 

1. A method for measuring thickness of a sample using an eddy current probe comprising of an absolute coil including the steps of: a) An AC voltage at a selected frequency is used to drive the sense coil and reference coil. As sense coil comes close to the surface of conductive sample, an eddy current is induced to conductive layer. An AC voltage meter measures both reactance and resistance at sense coil; b) Perform the measurement of eddy current sense coil at point of non-contact by using a capacitance sensor to determine distance ‘d’ of known thickness to generate the natural intersecting curve. c) Generate the digital calibration thickness versus X or Y amplitude curve to predict the unknown thickness. This will last for the entire life of the instrument without having to recalibrate again by eliminating the offset coefficient ‘b’, which comes from thermal drift or differences from eddy current probe to eddy current probe. This also allows the application of multiple eddy current probes either by simultaneous or multiplexing technique. d) Use different material to make calibration standard curve to measure the thickness of a different material.
 2. A side mount absolute eddy current probe produces magnetic field lines parallel to surface sample as a result of the eddy current path run normal to surface of conductive layer. In other words, the eddy current path that runs from the top surface conductive layer to the bottom layer then comes back to the top surface.
 3. A capacitance probe adjacent to eddy current housing to ensure non-contact at constant ‘d’ distance between eddy current coil and surface of known thickness samples. 